1. Field of the Invention
The present invention relates to semiconductor optoelectronics including devices for photodetection, optical modulation and switching, emission of non-coherent or coherent radiation.
2. Description of the Prior Art
In semiconductor structures, the tunneling of electrons across a potential barrier may be accompanied by the emission of optical (both visible and IR) radiation. In particular, tunneling through the barrier in the p-n junction may result in emission of photons with an photon energy controlled by the junction voltage (so called xe2x80x9cdiagonal tunnelingxe2x80x9d). An interesting feature of the emission is the feasibility of tuning the wavelength by application of a voltage. Progress in fabrication of quantum wells is opening new opportunities for the use of tunneling in optoelectronic devices, both for radiation sources, such as lasers, and detectors.
In R. F. Kazarinov and R. A, Suris, xe2x80x9cPossibility of the amplification of electromagnetic waves in a semiconductor with a superlattice,xe2x80x9d Sov. Phys. Semicond, 5 (10), 207-209, (1971, October), the optical amplification in the IR region is suggested by transitions between electronic minibands in semiconductor superlattice. Different other versions for unipolar laser operation are proposed in following papers: E. M. Belenov, P. G. Eliseev, A. N. Oraevskii, V. I. Romanenko, A. G. Sobolev, and A. V. Uskov, xe2x80x9cAnalysis of optical amplification due to tunneling of electrons in a quantum-well semiconductor structurexe2x80x9d, Sov. J Quant. Electronl, 18 (8), 995-999 (1988, August). R. Q. Uang and J. M. Zu, xe2x80x9cPopulation inversion through resonant interband tunnelingxe2x80x9d, AppL. Phys. Lett., 59, 181-182 (1991, July 8). A. Katalsky, V. J. Goldman, and J. H. Abelesxe2x80x9d xe2x80x9cPossibility of infrared lasaer in a resonant tunneling structurexe2x80x9d, Appl. Phys. Lett., 59 (21), 2636-2638 (Nov. 18, 1991). Q. Hu and S. Feng, xe2x80x9cFeasibility of far-infrared lasers using multiple semiconductor quantum wellsxe2x80x9d, Appl. Phys. Lett., 59, 2923-2925 (Dec. 2, 1991). S. I. Borenstain and J. Katz, xe2x80x9cEvaluation of the feasibility of a far-infrared laser based on intersubband transitions in quantum wellsxe2x80x9d, AppL. S Phys. Lett., 55, 654-656 (Aug. 14, 1992). Other structures are proposed to provide the amplification by intraband (intersubband) transitions of electrons in quantum-well structures. In E. M. Belenov, P. G. Eliseev, A. N. Oraevskii, V. I. Romanenko, A. G. Sobolev, and A. V. Uskov, xe2x80x9cAnalysis of optical amplification due to tunneling of electrons in a quantum-well semiconductor structurexe2x80x9d, Sov. J Quant. Electronl, 18 (8), 995-999 (August, 1988) resonant tunneling was considered in a quantum well with initial and final states in continuous spectrum. It was proposed also to employ a series (cascade) of such single-step structures to increase the effective optical gain of electromagnetic wave passing through the cascade structure. It was shown that under some bias condition the optical gain of 100 cmxe2x88x921 can be obtained with a spectral peak tunable by the bias voltage. These results are claimed to be applicable to the emission sources (electroluminescent and laser diodes), photodetectors (wavelength-tunable selective photodetection) and optical modulators. Simplified energy band diagram of quantum-well tunnel heterostructure is discussed in E. M. Belenov, P. G. Eliseev, A. N. Oraevskii, V. I. Romanenko, A. G. Sobolev, and A. V. Uskov, xe2x80x9cAnalysis of optical amplification due to tunneling of electrons in a quantum-well semiconductor structurexe2x80x9d, Sov. J Quant. Electron., 18 (8), 995-999 (August, 1988).
In A. Kastalsky, V. J. Goldman, and J. H. Abeles, xe2x80x9cPossibility of infrared laser in a resonant tunneling structurexe2x80x9d, AppL. Phys. Lett., 59 (21), 2636-2638 (Nov. 18, 1991), a theoretical analysis was reported of a cascadable quantum-well tunnel structure and the magnitude of the gain of 50-90 cmxe2x88x921 was claimed in the photon energy range near 0.12 eV (wavelengthxe2x80x9410 xcexcm). The three-barrier scheme was assumed as a multi-layer period of the structure with quantum-wells separated by bulk regions.
Unipolar laser action was achieved in and reported by J. Faist, F. Capasso, D. L. Slvco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, xe2x80x9cQuantum cascade laserxe2x80x9d, Science, v. 264, pp. 553-556 (Apr. 22, 1994). The heterostructure based on InGaAs/AIInGaAs heterosystem were proposed for laser operation at 10.6 xcexcm with InP cladding layers and InP substrate. A basic structure included many periods of quantum-wells with a total thickness of 4 xcexcm. In this case, the calculated optical confinement parameter relative the combined active region was estimated as 0.78 and laser oscillation threshold gain obtainable in a 1 mm long diode was estimated as 22 cmxe2x88x921. The calculated efficiency of the proposed laser was stated as 1.3%. The key feature of the proposed structure was identified as the fact that electrons tunnel from the lower level of the active region faster than electron-phonon relaxation time which controls electron population of the upper level. Thus the resonant tunneling is proposed to work for emptying the final state of laser optical transition.
Experimental studies of miniband-transition absorption and quantum-well intersubband absorption were reported in L. C. West and S. I. Eglash, xe2x80x9cFirst observation of an extremely large dipole transition within the conduction band of a GaAs quantum wellxe2x80x9d, AppL. Phys. Lett., 46, 1156-1158 (Jun. 15, 1985). B. F. Levine, K. K. Choi, C. G. Bethea, J. Walker, and R. J. Malik, xe2x80x9cNew 10 xcexcm infrared detector using intersubband absorption in resonant tunneling GaAlAs superlatticesxe2x80x9d, Appl. Phys. Lett., 50, 1092-1094, (Apr. 20, 1987). J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, xe2x80x9cMeasurement of the intersubband scattering rate in semiconductor quantum wells by excited state differential absorption spectroscopyxe2x80x9d, Appl. Phys. Lett., 63 (10), 1354-1356 (Sept. 6, 1993) J. Faist, C. Sirtori, F. Capasso, L. Pfeiffer, and K. W. West, xe2x80x9cPhonon limited intersubband lifetimes and linewidths in a two-dimensional electron gasxe2x80x9d, Appl. Phys. Lett., 64 (7), 872-874 (Feb. 14, 1994)] and in a number of other papers, whereas opposite optical processes photon emission and gain were observed later by [M. Helm, E. Colas, P. England, F. DeRosa, and S. J. Allen, Jr., xe2x80x9cObservation of grating induced intersubband emission from GaAs/AIGaAs superlatticesxe2x80x9d. Appl. Phpys. Lett., 53, 1714-1716 (Oct. 3, 1998). J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, xe2x80x9cMid-infrared field-tunable intersubband electroluminescence at room temperature by photon-assisted tunneling in coupled.quantum wellsxe2x80x9d, Appl. Phys. Lett., 64 (9), 1144-1146 (Feb. 28, 1994). J. Faist, F. Capasso, C. Sirtori, D. L, Sivco, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, xe2x80x98Narrowing of the intersubband electroluminescent spectrum in coupled-quantum-well heterostructuresxe2x80x9d, Appl. Phys. Lett., 65 (1), 94-96 (May 3, 1994). J. Faist, F. Capasso, D. L. Slvco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, xe2x80x9cQuantum cascade laserxe2x80x9d, Science, v. 264, pp. 553-556 (Apr. 22, 1994)]. Most of mid-IR laser realizations of quantum-cascade tunneling lasers are associated with heterosystem InGaAs/AlInGaAs on the InP substrates [J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, xe2x80x9cQuantum cascade laserxe2x80x9d, Science, v. 264, pp. 553-556 (Apr. 22, 1994). J. Faist, F. Capasso, D. L. Sivco, A. L. Hutchinson, C. Sirtori, S. N. G. Chu, and A. Y. Cho, xe2x80x9cQuantum cascade laser: Temperature dependence of the performance characateristics and high To operationxe2x80x9d, Appl. Phys. Lett., 65, 2901-2903 (1994). J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, and A. Y. Cho, xe2x80x9cContinuous wave operation of a vertical transition quantum cascade laser above T=80K, Appl. Phys. Lett., 67 (21), pp. 3057-3069 (Nov. 20, 1995), J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, M. S. Hybersten, and A. Y. Cho, xe2x80x98Quantum cascade laser without intersubband population inversion,xe2x80x9d Phys. Rev. Lett., 76,411-414 (1996).
However, despite the work that has been done so far, there continues to be a need to provide unipolar optoelectronic nano-structure devices which have precisely enforced geometrical properties such as uniformity in sizes of the nano-technological objects in the devices, high accuracy in spatial shaping and distance positioning of the objects, providing of proper composition variations etc.
It is therefore an object of the present invention to provide a family of unipolar optoelectronic nano-structures: lasers, radiation-emitting (non-coherent) diodes, optical modulators, switches and optical non-linear components, photodetectors in new wavelength ranges, for nitride materials. The nitride-based nano-structures of the present invention can be as complicated as a multi-layer, including quantum wells and/or quantum dots, devices with precisely enforced geometrical properties such as uniformity in sizes of the nano-technological objects, high accuracy in spatial shaping and distance positioning if the objects, providing of proper composition variations etc.
According to one aspect of the present invention, there is provided a unipolar semiconductor structure comprising: at least one active layer comprising at least one group III-nitride; and at two barrier layers disposed on either side of the active layer, each of the two barrier layers comprising at least one group III-nitride.